1. Field of The Invention
This invention relates generally to the electrometallurgical melting of titanium and titanium-based alloys. In particular, it relates to the electroslag of titanium and titanium-based alloys, and more specifically to magnetically-controllable, electroslag melting of these metals.
This invention may find application in the production of titanium, and particularly low alloyed titanium-based alloys, characterized by a high density of cast metal, absence of gas pores and inclusions, and low contents of admixtures, thereby allowing application of the above materials in aviation and shipbuilding industries, power generation, chemistry, nuclear power sector and manufacture of home appliances.
2. Description of the Prior Art
Methods of electron beam and induction melting of titanium and its alloys, have been described in E. L. Morozov et al., "Investigation of Various Methods of Melting and Casting of Titanium Alloys", TMS/AIME, P.O. Box 430,420 Commonwealth Dr., Warrendale, Pa., 15086, 1980; D. J. Chronister, S. W. Scott. D. R. Stickle, D. Eylon, F. H. Froes, "Induction of Skull Melting of Titanium and Other Reactive Alloys", Journal of Metals, September 1986, pp. 51-54.
Common drawbacks of metals produced by these methods include low density and the presence of gas pores, cavities, and inclusions. Serious drawbacks exist in the chemical and physical heterogeneity caused by the low hydrodynamic activity of metallurgical melt in the course of crystallization.
A well-known and widely used commercial method of melting titanium and its alloys is the vacuum arc melting (VAM) method which is disclosed in U.S. Pat. No. 5,127,468. This technology is over fifty years old, and resources for improving the quality of the metal produced by this technology have been exhausted. Metals produced by this method are characterized by the presence of gas pores and inclusions, as well as a nonuniform distribution of alloying elements and admixtures. Arc burning and passage of an electric current through the metallurgical pool result in a certain hydrodynamic activity of the melt, which is higher than in cases of electron beam and induction melting. However, the flows existing in the metal pool have an unfavorable orientation, thereby making it deeper in the central area. As a result, an area of loose metal featuring concentration of gas pores and inclusions is formed in the axial portion of an ingot.
Electroslag processes possess extensive, but as yet unused, possibilities of producing chemically and physically homogeneous metal (see e.g. "Avtomaticheskaya Svarka", No.10, 1963, pp. 37-42, published by the Ukrainian SSR Academy of Sciences, Kiev and U.S. Pat. No. 3,989,091). In the melting oftitanium and its alloys, the known electroslag melting technology does not take into account the effect of magnetohydrodynamic processes inside slag and metal pools on crystallization of metal.
In contrast to other known technologies of electric melting of titanium and its alloys, melting with the use of electroslag processes involve the presence of a slag pool above the metal one. The presence of the slag pool featuring a high density of electric current has a wide range of hydrodynamic effects on metal crystallization. These effects may be both positive and negative. In the well-known processes of electroslag refining (ESR), the hydrodynamics of the slag pool (intense) and metal pool (less intense) is governed by electromagnetic forces arising from the interaction between the melting current, I.sub.m, and the proper magnetic field. The size of the electrode gap and melting current, I.sub.m, along with its value, density, and passage path in slag and metal melts constitute major reasons for the motion of a current-carrying fluid. In the above-mentioned studies, the electric current, which had been decreasing in the course of melting, was stabilized through the variation of the electrode feed rate.
The known electroslag melting processes do not include any mechanisms for stabilizing the crystal structure and ingot quality over its whole height through stabilization of the hydrodynamic situation in slag and metal melts. Permanent variations of hydrodynamics in the course of the melting process are caused by naturally decreasing the melting current, I.sub.m, as the electrode is melted. A decrease in the voltage drop across the electrode, U.sub.e, which is not registered in the course of melting, automatically increases the voltage drop across the slag pool, U.sub.s : U.sub.m =U.sub.e +U.sub.s =const.
Since in the known electroslag processes of titanium melting the value of U.sub.m is constant (it is picked up from terminals of the power supply secondary winding and is measured either on these terminals or on current leads of electrode and crystallizer), the value of U.sub.s, which is permanently increasing in the course of melting, results in a smooth decrease of I.sub.m. Here, current passage path and current density are changed as a result of the increase in the electrode gap. The only known method of stabilizing the value of I.sub.m, other than by the size of the electrode gap, consists of increasing the rate of electrode feed into the pool. The stabilization of the current value however, as in the case of the constant electrode feed rate, is accompanied by an increase in the electrode gap. As the voltage drops across the slag pool, U.sub.s increases. In so doing, the melting process is accompanied by permanent changes in the melting current path within the electrode gap, the corresponding magnetic field, and the bulk electromagnetic forces, causing toroidal motion of the melt. Therefore, in order to avoid violating the electroslag process at the end of melting by electrode withdrawal from the slag pool and arc burning at the slag surface, the initial stage of melting is carried out at a minimal sized electrode gap. From the standpoint of hydrodynamics, these are the most unfavorable conditions of ingot crystallization because of formation along the ingot axis of an area of low-quality, loose metal containing gas pores and inclusions. In addition, the initial stage of melting is characterized by nonuniform distribution of alloying elements and admixtures in the metal. Electrode end face lift to the slag pool surface results in the increase of slag pool involvement in the toroidal motion of the melt, increase in metal homogeneity, and elimination of the low-quality metal area along the ingot axis. The Most favorable conditions for ingot crystallization are created by a stable electroslag melting of the electrode next to the slag pool surface.
The prior art melting processes do not permit the stabilization of the electrode melting mode in the upper layers of the pool with a constant electrode gap. As the electrode "exits" to the slag pool surface an unstable electric arc occurs at the slag pool surface, accompanied by sputtering (slopping) of slag and inadmissible low quality of ingot formation.